FIG. 16 is an exploded perspective view of conventional common mode noise filter 1001 disclosed in Japanese Patent Laid-Open Publication No. 2000-190410. Conductors 2A, 2B, 3A and 3B having spiral shapes containing metal are provided on top surfaces of insulating layers 1A to 1D, respectively. Conductors 2A and 2B are connected to each other with via-electrode 4A, providing coil 2. Conductors 3A and 3B are connected to each other with via-electrode 4B, providing coil 3. Insulating layers 5 made of magnetic material are provided on a bottom surface of insulating layer 1A and on the top surface of conductor 3B, respectively. Insulating layers 1B to 1D are made of nonmagnetic material. Insulating layers 1A and 5 are made of magnetic material. Conductors 2B and 3A facing each other across insulating layer 1C are magnetically coupled to each other. This arrangement allows coils 2 and 3 to have large impedance for common mode components of signals flowing through these coils, thereby eliminating noises in the common mode components.
In common mode noise filter 1001, if insulating layer 1C is thin, insulating layer 1C may cause insulation failure or metal migration between conductors 2B and 3A, that is, between coils 2 and 3 facing each other across insulating layer 1C. If insulating layers 1B and 1D are thick, the distance between insulating layer 1A and conductor 2B, and the distance between insulating and conductor 3A and layers 5 provided on a top surface of conductor 3A are too long to efficiently utilize magnetic fields generated in insulating layer 1A and insulating layers 5. This arrangement may cause coils 2 and 3 not to have large impedance against the common mode components.
FIG. 17 is an exploded perspective view of conventional common mode noise filter 1002 disclosed in Japanese Patent Laid-Open Publication No. 2001-76930. Conductors 502A, 502B, 503A, and 503B are provided on top surfaces of insulating layers 501A to 501D, respectively. Conductor 502A and conductor 502B having spiral shapes are connected to each other with via-electrode 504A, providing coil 502. Conductors 503A and 503B are connected to each other with via-electrode 504B, providing coil 503. Insulating layer 501E made of magnetic material is formed on a top surface of conductor 503B. Magnetic portions 505 are formed on insulating layers 501B to 501D made of nonmagnetic material and inside the spiral shapes of conductors 502B and 503A.
Coils 502 and 503 have large impedance against common mode components of signals flowing through these coils, thereby eliminating common mode noises. Magnetic portions 505 increase magnetic field between coils 502 and 503, and increase the impedance of coils 502 and 503 against the common mode components, thereby further eliminating common mode noises.
Conventional common mode noise filter 1002 includes magnetic portions 505 respective ones of which are provided on insulating layers 501B to 501D made of nonmagnetic material. Magnetic portions 505 are provided by forming holes in insulating layers 501B to 501D, filling the holes with magnetic material paste, and hardening the paste thermally. In order to increase the sizes of magnetic portions 505, the holes for forming magnetic portions 505 need to have large diameters. The increasing of the diameters of the holes causes the amount of the magnetic material paste to be larger than the volumes of the holes since the paste are easily influenced by surface tension. This may produce spaces around magnetic portions 505, and hence, reduce the magnetic field between coils 502 and 503, thus preventing coils 502 and 503 from having large impedance against the common mode components.